Cerium oxide has an oxidation-reduction potential of as low as about 1.6 V between Ce4+ and Ce3+, and hence the reaction represented by the formula below proceeds reversibly. For this reason, cerium oxide exhibits an oxygen storage capacity (hereinafter referred to as OSC), and is thus used as a promoter in an automotive three-way catalyst or as a catalyst support.CeO2CeO2-X+X/2O2(X=0 to 0.5)
However, pure cerium oxide is well known for having an extremely low OSC, i.e., X=about 0.005.
To improve this, there have been numerous reports that state that, for example, by forming a solid solution of zirconium oxide in cerium oxide, (1) the heat resistance of the specific surface area of cerium oxide can be improved, and (2) the OSC can be improved by allowing insertion of Zr4+ having a small ionic radius into a Ce backbone to alleviate the increase in volume during the above-mentioned reaction.
Patent Literature 1 discloses a cerium/zirconium-base composite oxide containing cerium and zirconium, wherein the composite oxide satisfies requirements that (1) an oxygen release initiation temperature is 380° C. or below, that (2) an oxygen release amount is 485 μmol/g or more, and that (3) an oxygen release amount at 400° C. is 15 μmol/g or more.
This composite oxide containing cerium and zirconium is obtained by mixing a cerium starting material containing cerium oxide and a zirconium starting material containing zirconium oxide at a predetermined mixing ratio; melting the resulting starting material mixture at a temperature at or above the melting point; cooling the melt to form an ingot; grinding the ingot to prepare a powder; then optionally removing strain within powder crystal grains under heating; and grinding to a further fine state. As such, unlike the composite oxides containing cerium and zirconium that can be obtained by a usual wet process, the composite oxide of Patent Literature 1 is obtained by a melting process (dry process), and thus has the above-mentioned unique properties.
Since the mixture is slowly cooled after the melting has been performed once, a final product can be obtained with a good crystallinity and with a large crystallite diameter. When considered based on a particle unit, such an increase in the crystallite diameter indicates that the number of atoms within the structure (i.e., the internal energy) is increased, relative to the number of atoms on the surface (i.e., the surface energy), causing deactivation of the surface. That is, in an energy base, it is in a state where the oxygen in the bulk easily moves, whereas the oxygen in the surface cannot easily move.
The above demonstrates that there is a limitation in improving OSC at a temperature lower than the above.
Patent Literature 2 discloses a cerium oxide-zirconium oxide-based mixed oxide comprising cerium oxide and zirconium oxide, wherein (1) the weight ratio of CeO2:ZrO2 is 60:40 to 90:10, and (2) the cerium oxide and the zirconium oxide are present as a mixture, the zirconium oxide being composed of a solid solution in which tetragonal or cubic zirconium oxide contains cerium oxide. This composite metal oxide is excellent in platinum dispersibility; however, despite such an advantageous feature, it suffers from an insufficient OSC at a low temperature.
Patent Literature 3 discloses a process for producing a metal oxide particle comprising a core part relatively rich in a ceria-zirconia solid solution and a surface layer relatively rich in a second metal oxide, the process comprising: providing a sol containing at least a population of ceria-zirconia solid solution colloid particles and a population of second metal oxide colloid particles differing in the isoelectric point with each other; adjusting the pH of said sol to be closer to the isoelectric point of said population of ceria-zirconia solid solution colloid particles than to the isoelectric point of said population of second metal oxide colloid particles, thereby aggregating said population of ceria-zirconia solid solution colloid particles; adjusting the pH of said sol to be closer to the isoelectric point of said population of second metal oxide colloid particles than to the isoelectric point of said population of ceria-zirconia solid solution colloid particles, thereby aggregating said population of second metal oxide colloid particles onto said population of ceria-zirconia solid solution colloid particles aggregated; and drying and firing the obtained aggregate. However, this production process requires complicated operations, such as the use of ceria-zirconia solid solution colloid particles and second metal oxide colloid particles. Further, the resulting metal oxide particle suffers from an insufficient OSC at a low temperature.